PRINCIPLES FOR THE SUSTAINABLE UTILIZATION OF THE OKAVANGO DELTA ECOSYSTEM, BOTSWANA

A broad understanding of the structure and functioning of the Okavango Delta ecosystem has provided a basis for evaluating the kinds of pert urbations that are likely to affect the ecosystem as a whole. Sediment introduced into the system results in constant changes in the distrib ution of water on the fan surface. This promotes the occurrence of a v ariety of habitats in different stages of wetting and drying, and acco unts for the overall habitat diversity in the system. It also promotes regeneration of saline soils that are locally toxic to vegetation on islands in the Okavango Delta. Water abstraction from the lower reache s of the fan by activities such as dredging may be rendered useless by changes in the distribution of water on the fan over relatively short time spans. However, water abstraction in itself is not necessarily a problem. Ideally it should be none fr om the apex of the fan, provide d it is small relative to the total inflow (1-2%), and does not disrup t sediment supply to the system from source areas, such as by the cons truction of a weir ol dam. An additional impact of the construction of an impoundment in the catchment would be an. increase in the total di ssolved solid concentration of inflowing water. The system is adapted to low total dissolved solid concentrations and, by affecting the natu re of plant communities at the apex of the fan, the system as a whole could be affected. The dominance of transpiration over evaporation in this wetland ecosystem results in the accumulation of dissolved substa nces, notably silica and calcium and magnesium carbonate below surface , where they are biologically not deleterious. Sustained removal of ve getation may result in salinization of surface water, and would have a large impact on the ecosystem. Additional impacts that could alter th e structure and functioning of the ecosystem include eutrophication th at may result from agricultural development in the catchment, which ma y profoundly affect the nature of vegetation communities in the upper reaches of the fan, and thus the patterns of sediment and water dispel sal. We no not regard the present utilization of al eas around the pe riphery of the Delta as a direct threat to the ecosystem itself.